Brewz, I finally found my old HD-5 carb from the 029. I checked it out and remembered I had drilled out the limiter jet in the carb, but I hadn't gotten around to comparing it to the 460 carb. Anyway, I fired up the saw with the modded HD-5, got the idle worked out and then tried to rev it. It wouldn't go past 11,000 rpm, so I went back in and blocked off the auxiliary jet for the high speed needle.
I had put the Works Connection tach on the saw so I could see what rpms it was running. I set it to 13,000 WOT and went cutting wood. The saw will torque down to below 6,000 without bogging and likes to throw big chips at those lower rpms. I tweaked the high speed needle a bit leaner while out cutting.
The stock HD-5 on an 029 (54cc) is supposed to be set to 13,000. The same carb on an 039 (64cc) is suppose to be set at 12,500. The tuning sounds counter intuitive, but what is happening is that on the bigger displacement engine the high speed fuel curve needs to be brought down lower in the rpm band to feed the torque peak. When the limiter is removed and the low speed circuit flowing more, the high speed needle can be leaned back out.
Besides having advanced the ignition timing about 6 degrees, I also built a high-flow air filter for the saw. It uses the old 029 filter (without the compensator port) and the cage has been drilled all the way around. I then glued some thin filter foam around the cage and then sewed a cover around it using an old flannel shirt. Works great.
While out cutting I got the saw good and hot and then ran it WOT until the rpm stabilised, it showed 13,350 on the tach. I then took off the high-flow filter and put on a fresh stock filter (the compensator port was plugged) and did the same experiment. With the stock filter it showed 13,275. It may have only been around 75 rpm difference, but I hadn't cleaned the high-flow filter for the last three times I went cutting - and it still breathed better than a fresh, never used, stock filter.
In other words, running a bigger venturi on one of these saws isn't going to help - the filter is the bottle neck.
The 460 carb not only has a bigger venturi, but the discharge nozzle position is different. The fuel curve for the high speed circuit will be different to the HD-5. However, I think if you leaned out the high speed needle on the 460 carb the fuel curve should flatten out. I think you may be running it too rich with the WOT rpm down at 12,700-800 (that rpm works for a stock HD-5 carb)
Here's something else I noticed, the HD-5 has a stronger metering spring than the 460 carb. In other words, there is a lot of potential in the 460 carb for bigger engines. It has much bigger air bleed/transition holes to start with - put in a stronger metering spring and the low speed circuit could probably flow enough for a 90cc engine. With the bigger venturi and the potential in the low speed circuit, it seems a more logical choice than trying to adapt a WJ to something like a big bore 372.
Now that I have the HD-5 modded, I'll have to run a test to see which carb I prefer.
